Laser, such as for example solid state lasers, use man-made doped monocrystals, i.e. single crystals, and glasses as active laser media. Single crystals and glasses may be selected to be transparent in a predetermined wavelength region of the electromagnetic spectrum. In addition, single crystals exhibit good thermal conductivity, while glasses can be formed in any shape and size.
However, the known active laser media and the method for producing the same suffer from a number of disadvantages. On one side, glasses have a poor thermal conductivity, thereby limiting the average power of the lasers using glasses as active media. For example, glasses used as active laser media within mega joule lasers limit their use to an operation at a few pulses per day, typically 3 or 4 pulses per day.
On the other side, the known methods for producing single crystals are very slow, tedious and expensive. For example, the known methods for producing doped single crystals involve macroscopic segregation of a dopant during the growth process of the single crystals in a liquid-solid equilibrium. As a consequence, not only the maximum concentration of a dopant within a single crystal is limited, but also any doped single crystal obtained by such a method exhibits a dopant concentration gradient. In addition, the step of melting the material requires the use of expensive high temperature equipments (e.g. platinum, iridium crucibles), which may pollute the single crystals.
A further disadvantage of single crystals is that, at a macroscopic level, in order to produce large single crystals, the crystal growth is slow, e.g. 10-100 μm per hour during fusion of the materials, e.g. at a temperature of 1000° C.-2500° C. Consequently, days are required to grow single crystals. At a microscopic level, the crystal growth is fast, when compared to natural crystals. Consequently, the quality of a man-made single crystal remains poor due to the presence of a large amount of crystal defects. Consequently, only a middle portion of a single crystal is usable as active laser medium, the head and the tail of the single crystal as well as the remaining liquid phase being discarded. In a typical case, about 30% of the material may be lost in the crucible during crystal growth, and less than 50% of the resulting single crystal is considered to be usable as active laser medium.
A further disadvantage is that birefringence effects may be observed using a single crystal as active laser medium, thereby depolarizing the laser beam.
Further, single crystals are quite fragile. For example, single crystals have a limited thermomechanical resistance and fracture strength. Therefore, cracks within a monocrystalline structure can easily propagate.
Not only single crystal growth techniques are complex and energy intensive, but also the optical homogeneity of a single crystal is limited. As a result, wavefront propagation distortion remains and ever occurring problem.
The use of transparent ceramics as active laser media has been described by Hatch, Parsons and Weagley (Appl. Phys. Lett. 5(8) (1964) 153-154). According to these authors, transparent ceramics may have properties approaching the properties of single crystals. However, the method used for producing these ceramics has still a number of disadvantages. For example, the method is very energy-intensive as it comprises subsequent steps of vacuum hot-pressing and annealing. In addition, the method requires the synthesis of single crystals. Also, the obtained materials exhibit poor transparency and high level of optical loss in short wavelengths in the visible domain.
As of today, the most studied ceramic materials having potential laser application are oxide based transparent ceramics. For example, an yttrium aluminum garnet (YAG) ceramic was reported in 1995 by Ikesue et al. (J. Am. Ceram. Soc. 78(4) (1995) 1033). However, the known oxide based ceramics are synthesized through a tedious method under extreme conditions, in terms of temperature, which may exceed 1650° C. Typical synthetic methods described by Ikesue et al. require the production of Y3Al5O12 to occur during a high temperature sintering step by reaction of alumina (Al2O3) with yttrium oxide (Y2O3). More specifically, a stable aqueous suspension comprising a mixture of Al2O3, Y2O3, and adjuvants (deflocculants, binding agents and pH modifiers) is first obtained by slow, mild and long stirring (12 h) using a ball milling apparatus. This milled slurry is then spray dried to obtain highly compact polycrystalline aggregates having an average size in the micron range such as lower than 150 microns. These aggregates are then cold isostatically pressed (CIP) (98-200 MPa) to constitute a “green body”, i.e. an unsintered ceramic material, before debinding and calcinating under air to remove the adjuvants. The resulting material is then sintered for 20 hours at 1750° C. under high vacuum. After the sintering step, additional steps of hot isostatic pressing (HIP) and annealing are then performed to improve density and remove electronic defaults induced during the high vacuum sintering, respectively. Since this initial article, it is worth noting that the group of Ikesue has managed to bypass the spray drying step by directly casting or casting under pressure the milled slurry using a porous mould.
Another exemplary method for providing YAG ceramics has been provided by Konoshima Chemical Co., Ltd (Journal of Alloys and Compounds 341 (2002) 220-225). The method consists of preparing a YAG phase through calcination at 1200° C. of a precursor gel. First, a mixture of powders is provided. Then, a slurry is prepared by adding solvents, deflocculants, binding agents and pH modifiers to the powders, and ball milling the resulting mixture for 24 h. This mixture is then cast into a porous mould (typically gypsum) to obtain a “green body”. After a further drying step, the resulting material is removed from the mold, calcinated under high temperature, and then sintered under high vacuum for 20 hours at 1750° C. After the sintering, additional steps of hot isostatic pressing (HIP) and annealing are performed to remove electronic defaults induced during the sintering under high vacuum. Another exemplary method for providing YAG ceramics has been provided by Rabinovitch et al. (Optical Materials 24 (2003) 345-351), the method described being quite similar to the ones described by Ikesue et al. and Konoshima Chemical Co., Ltd.
While the transparency window of known oxide based ceramics remains generally quite narrow, fluoride based ceramics may have a certain transparency within a larger range from 190 nm to 7 μm. Furthermore, metal fluoride ceramics may exhibit thermal conductivity comparable to the thermal conductivity of single crystals. In addition, metal fluoride ceramics may possess improved hardness, breaking strength and thermal shock properties. Further, metal fluoride ceramics may be produced in any shape and size. Also, methods for producing metal fluoride ceramics require shorter times and lower temperatures than those required in single crystal growth processes. However, the transparency of metal fluoride ceramics is insufficient for laser applications because no practical method for producing fluoride based ceramics having optical properties meeting the requirements of active laser media has yet been described. Indeed, since the work of Hatch, Parsons and Weagley, methods for producing transparent metal fluoride ceramics for laser applications have been quite unsuccessful because most metal fluoride ceramics have an unacceptable amount of optical defects, e.g. defects localized in grain boundaries or pores, defects due to dopant segregation or grain scale heterogeneity, etc. These defects render the known metal fluoride ceramics unsuitable as active laser media.
As a first example, Basiev et al. (Optical Materials 35 (2013) 444-450) describe the use of single crystals for the synthesis of CaF2:Yb ceramics obtained by uniaxial hot pressing or hot-forming dry materials under vacuum. Although lasing results were observed, optical defects were still present such as Yb2+ impurities. In addition, the methods proposed by these authors still require high-energy input and preparation of single crystals to merely provide ceramics having a layered structure.
As a second example, a preparation of fluoride based ceramics has been described by one of the inventors of the present application (Optical Materials 34 (2012) 965-962). However, the method described therein requires the use of dried powders, which are then annealed, pressed and sintered under vacuum. The obtained ceramic must then be post-treated by uniaxial hot pressing to obtain transparency. In addition, optical defects were still present and thus no slope efficiency results were observed.
As a further example, a method for providing CaF2:Yb,La ceramics has been described by Nikon Corporation (Advanced Solid-State Lasers Congress Postdeadline Papers, OSA 2013, JTh5A7) with somehow decent results but with at least two major inconvenients. First, the method implies codoping with optically inactive La3+ ions, thereby reducing the thermal conductivity of the ceramic. Second, the method requires a standard sintering step including pressing and sintering dry powders. In addition, the use of LaF3 involves the substitution of Yb3+ ions by La3+ ions, which alters the structure of Yb6F37 hexameric clusters, thereby producing optical defects and insufficient optical properties.
As a further example, a method for providing Nd-doped CaF2 ceramics has been described by Gang Lu et al. (Materials Letters 115 (2014) 162-164). The method described involves uniaxial hot-pressing under vacuum (900° C., 30 MPa) of dry particles obtained by coprecipitation. However, the resulting ceramic is of poor quality and has a limited transparency.
In view of the above, standard methods for providing metal fluoride ceramics involve a main step comprising the pressing of a dry powder prior to the sintering. However, the pressing of a dry powder generates formation of aggregates, especially when the powder has an average particle size in the nanometric range. As a result, during the sintering, the presence of aggregates generates differential sintering which is a source of porosity and thus of poor transparency. Furthermore, not only the sintering step must be performed under constraints, but also additional post-sintering steps under constraints are also typically required.
Accordingly, there exists a continuing need to develop improved active laser media and methods for producing the same.